Non-patent Document 1 shows an InGaN/GaN quantum well structure made on a GaN (11-22) substrate. An off angle of the GaN substrate is 58 degrees formed in the a-axis direction. Since a cavity structure cannot be formed with the above-described crystal facets, the quantum well structure is made to emit light by photo-pumping. Pumping light is guided in a direction along a plane defined by the c-axis and a-axis. This quantum well structure is disposed between GaN optical guide layers. An emission measured along the [-1-123] direction is generated with a lower energy and a lower threshold pumping power than an emission measured along the [1-100] direction.
Non-patent Document 2 discloses laser diodes made on an m-plane. The laser diodes have the InGaN/GaN quantum well structure and the lasing wavelength thereof is 400 nm. This quantum well structure is disposed between GaN guide layers. Stripes parallel to the a- and c-axes show TE mode operation, and the c-axis stripe laser diode among such laser diodes exhibits the lower threshold current densities. Non-patent Document 3 shows theoretical calculation of polarization degree.